1. Field
Aspects of the present invention generally relate to an image forming apparatus, such as a copier and a printer, that generally employs an electrophotographic method.
2. Description of the Related Art
A color image forming apparatus employing an electrophotographic method includes image forming units that are independently arranged for respective colors to accelerate print processing. Images are sequentially transferred from the image forming units of the respective colors to an intermediate transfer belt. Then, the images are collectively transferred from the intermediate transfer belt to a transfer medium.
In the color image forming apparatus with such a known configuration, color misregistration (misregistration of toner images of the respective colors) can occur due to mechanical factors provided in the image forming units of the respective colors. The color misregistration occurs when the images are superimposed. Particularly, in a case where laser scanners (optical scanning devices and exposure devices) and photosensitive drums are independently arranged relative to the image forming units of the respective colors, positional relations between the laser scanners and the photosensitive drums differ on a color basis. Consequently, laser scanning positions on the photosensitive drums cannot be synchronized, causing the color misregistration.
Among factors in the color misregistration, one of the most influential factors is fluctuations in laser irradiation position on the photosensitive drum due to thermal deformation of an optical unit. Generally, the optical unit deflects a laser beam emitted from a light source with a rotating polygon mirror to perform scanning. Between the emission of the laser beam from the light source to the arrival of the laser beam to the photosensitive drum, an optical element allows the laser beam to reflect on mirrors for several times to change a travelling direction of the laser beam, or adjusts a spot and a scanning width of the laser beam via a lens. Such an optical element determining an optical path for the laser beam is fixed to a frame of the optical unit. In a case where the frame is thermally deformed due to a temperature rise with operations of the image forming apparatus, a position of the optical element changes. This affects a direction of the optical path. Since a change in the optical path direction increases in proportion to a length of the optical path to the photosensitive drum, such a change appears as the fluctuations in the irradiation position even if the frame deformation of the optical unit is extremely small.
Such an image forming apparatus performs color misregistration correction control to correct the color misregistration. Japanese Patent Application Laid-Open No. 7-234612 discusses color misregistration correction control in which a detection toner image of each color is transferred from a photosensitive drum to an image bearing member (e.g., an intermediate transfer belt), and relative positions of the detection toner image in a scanning direction and a conveyance direction are detected by an optical sensor. Accordingly, the color misregistration is controlled based on the relative positions detected by the optical sensor.
Moreover, Japanese Patent Application Laid-Open No. 2000-218860 discusses laser irradiation position correction in which a temperature rise inside an image forming apparatus or a temperature of an optical unit is detected by a temperature sensor and a laser irradiation position is corrected by a correction controller.
According to each of such kinds of color misregistration correction control, however, the detection toner image is transferred from the photosensitive drum to the image bearing member (e.g., an intermediate transfer belt). This degrades usability of the image forming apparatus in terms of toner consumption and time necessary to clean the image bearing member, for example. Accordingly, Japanese Patent Application Laid-Open No. 2012-141587 discusses color misregistration correction control in which fluctuations in current value are detected, the current value fluctuations occurring when a detection electrostatic latent image formed on a photosensitive drum reaches a process unit. Thus, the color misregistration is controlled based on the detected current value fluctuations.
However, in the image forming apparatus forming the detection electrostatic latent image on the photosensitive drum to perform the color misregistration correction control (adjustment of light emission timing of an exposure device), the photosensitive drum is repeatedly exposed to the light for long periods. This may deteriorate sensitivity due to light-induced fatigue, and cause generation of a poor-quality image such as a decrease in image density.
Moreover, in a high-speed compact color image forming apparatus, thermal deformation of an optical unit is more noticeable. Thus, frequency of color misregistration correction needs to be increased. This causes difficulty in extending life span of a photosensitive drum.